"Not yet… this isn't enough. If I can use this energy freely, then…"
Leon decided to refer to the afterlife as the void, and if in the void where the place where souls would eventually crumble away and disappear, perhaps he could find some souls there. Just like his own soul had memories, the others would have as well, and he would use them. The creator of the system slaughtered billions of monsters and peoples after summoning them to the final battle, so it was obvious that they should have arrived at the same time, if not a little earlier than Leon.
Expanding his energy, Leon began to look for those souls. Yet, even though his energy could expand quite a lot, it had a limit, and within that limit, Leon didn't find anything. The void was the place beyond the physical realm, so it was only obvious that it was infinite. Even if billions of monsters and people were forced to go there after death, it doesn't mean that finding them would be easy.